simonton window balance mystery

I was attempting to repair a Simonton vinyl single hung window. The right side balance had come loose and was sitting in the window channel. The window still worked but was not smooth. I removed the sash and took out the balance. The balance appeared to be broken and a search for a replacement led me to your website. I am sending pictures of the balance and hope you can supply me with the right one. The mystery is how the balance worked in the first place. There is no balance stop in the window channel. It is completely smooth (both sides). There is nothing to hold the balance in place. I couldn't even put the old balance back in because there there was nothing there to hold it in place. Any information you could give me would be greatly appreciated .